1. Field of Invention
This invention relates generally to methods and apparatus suitable for use in measuring and detecting certain pupillary size and response parameters. More particularly, the invention relates to such methods and apparatus which, while suitable for use in pupilometers and other opthamological instruments in general, is particularly suitable for use in portable and hand-held pupilometers.
2. Description of Prior Art
It is known that the response of an individual's pupil to external light stimulation yields useful information as to the subjects well being. Pupillometry is an effective and non-invasive means of characterizing a subject's pupillary response and is used to determine the condition of the subject's nervous system.
Pupillometry has been the subject of focus for applications such as diagnosing Alzeheimer's disease (see e.g., Scinto et al., U.S. Pat. No. 5,704,369, Non-Invasive Method for Diagnosing Alzeheimer's Disease in a Patient; and Carter, U.S. Pat. No. 5,646,709 Portable Hand-Held Pupillometer with Dynamic Electronic Image Centering Aid and Method of use Thereof), detecting fatigue as indicated by an individual's alertness or sleepiness, and detecting the presence of alcohol and chemical/substance abuse (see e.g., Carter, U.S. Pat. No. 5,187,506, Method and Apparatus for Determining Physiological Parameters Based on Pupil Response), for detecting other conditions related to nervous system impairment, and for other ophthamological diagnostic applications(see e.g., Cornsweet, U.S. Pat. No. 5,042,937, Optical System for an Opthamological Instrument for Examination of Pupillary Responses; and Zanecchia et al., U.S. Pat. No. 5,534,952, Pupillometer). Thus, pupilometers, and the method associated therewith, may be useful in the medical, transportation, and military and law enforcement fields, in industry, and in other areas in which it is desirable to detect such conditions.
For these and other applications, it is desired to monitor the time-response of the individual's pupil as the eye is subjected to various lighting conditions. As an example, such a device would measure the pupil response of a dark-adjusted eye that is subject to a stimulating light pulse (i.e. photostimulus). For such an example, the pupil, initially large due to the dark-adjusted state, will typically decrease in size in response to an external light stimulation flash, and then increase in size upon returning to a dark-adjusted condition.
Briefly, conventional pupilometers typically include optical-electronic apparatus for generating a pupillary response-inducing light stimulus, and for measuring the diameter of the stimulated pupil over a period of time to establish the response (including parameters such as pupil constriction velocity, initial, minimum and final pupil diameter, time to minimum, and reflex amplitude) of a user/subject's pupils to the light stimulus. To this end, conventional pupilometers typically include one or more visible light emitting diodes to produce the response-inducing light stimulus (i.e., diodes to generate a visible flash of light directed along an optical path and at the subject's eye to cause contraction of the subject's pupil), and infrared diodes or other IR source and associated optics and electronics adapted to direct the IR source to and from the subject's eye for measuring the dynamic pupillary response to the light stimulus.
Prior methods and apparatus for measuring pupillary response generally utilize either a pupil imaging technique or a light scattering detection technique.
Imaging methods and apparatus rely on imaging the eye (or a portion thereof) on a detection device such as charged-coupled device (CCD), or other optical detector array. In these instances, the image of the eye, or a portion thereof, is typically detected on a two-dimensional detection device or a scanning one-dimensional device. The output of the imaging device is processed to determine the size of the pupil or other desired pupillary response data. Examples of such imaging apparatus and methods are disclosed in Carter, U.S. Pat. No. 5,187,506; Carter, U.S. Pat. No. 5,646,709; and Carter, U.S. Pat. No. 5,661,538, Portable Self-Measurement Pupillometer with Active Opto-Electronic Centering Aid and Method of use Thereof.
Among the known prior pupil-imaging pupillometer techniques, further distinction can be made by the imaging illumination or detection method. Techniques for both iris illumination (see e.g., Gardner et al., U.S. Pat. No. 4,850,691, Method and Apparatus for Determining Pupillary Response Parameters) and retinal scatter are utilized. For the former, the image of the pupil appears darker than the surrounding iris due to the higher reflectance from the iris for the monitoring illumination. For the latter technique, the pupil is illuminated and the image of the eye is similar to that commonly know as `red eye` in the photography industry. In this instance, the image of the pupil appears brighter than the surrounding iris due to the high scattering off the retina for the monitoring illumination conditions. Unfortunately, both of these prior imaging techniques suffer from difficulties in providing for proper alignment between the subject's eye and the detection device during response measurement.
Scattering pupilometers rely on the light scattered from the subject's eye under illumination from the monitoring illumination source. This scattered light is detected by a photo-detector element which converts the detected optical power to an electrical signal. The electrical signal output from the detection circuit is representative of the optical power incident on the active area of the photo-detector.
For example, the apparatus disclosed in Gardner et al. utilised a scattering technique which illuminates both the iris and the pupil area of the eye with the monitoring illumination. In this case, the detected signal is proportional to the scattered light from the illumination area. The output signal from the detector consists of a signal portion resulting from the illumination detected scattering from the iris, and a signal portion resulting from the illumination detected scattering from the retina. With the apparatus of Gardner et al. the scattering of the illumination light from the iris is greater than that from the retina, the major component in the output signal thus resulting from the scattering from the iris. Therefore, as the pupil contracts under light-constricting illumination, the output signal from the detector increases since the pupil decreases and the illuminated portion of the iris increases. A difficulty with this approach, as well as with other prior scattering-type pupilometers, is that the technique measures only relative pupil response; prior method and apparatus do not exist for absolute measurement of the pupil diameter during pupillary response measuring techniques.
In addition to the above-mentioned deficiencies in the prior art, there is a lack of practical pupillary response detection devices that will provide for a cost-effective imaging pupillometer, and particularly one that is relatively simple and compact, and thus suitable for use in portable, hand-held pupilometers.